Steam sterilization of autoclavable pelleted rodent diets causes increased pellet hardness which may result in prolonged intervals between litters, reduced weight gain, and reduced litter size. Very little is known about the optimum physical characteristics of rodent diets. Feed pellets from different diets as well as pellets from the same diet formulation manufactured by different rodent diet vendors may vary in size, shape, texture and pellet hardness. Normal pellet hardness measurements of commercially available rodent diets have not been reported. A test stand with a flat horizontal receptacle base and a DFI- 500 gauge load cell with a large round-flat upper test jaw (12.7 mm diameter) were used to collect pellet hardness measurements (kg force). Test diets were purchased from rodent diet vendors A, B, C and D. Autoclavable diets were autoclaved at 250xF at 15-18 lbs. pressure for 20 minutes. Results from 90 pellets (3 replicates of 30) from each mill date for each diet were compared for pellet hardness. Moisture content was also determined for each diet. Moisture content was similar for both nonautoclavable and autoclaved diets. It was concluded that: 1) Rodent diets significantly differ in pellet hardness, 2) Pellets from the same diet processed by different rodent diet vendors significantly differ in pellet hardness, 3) Steam sterilization of diets increases pellet hardness, and 4) In order to compare pellet hardness measurements from different diets, a standardized procedure must be used and the diet~s vendor specified.